The present invention relates generally to luminescent nanoparticles and methods for their preparation.
Fluorescence is the emission of light by a material when the material is excited by an external excitation source, and is a widely used tool in chemistry and biology. Materials that fluoresce are commonly referred to as phosphors. When light emitted by a phosphor persists for a perceptible duration of time after excitation ceases, i.e., for about 0.1 second or longer, the phenomenon is called phosphorescence.
Recently emerged nanoparticulate fluorescent technology has launched a new era for the development of fluorescent labels using inorganic complexes or particles. These inorganic materials offer substantial advantages over organic dyes, including a longer half-life, a broad excitation spectrum, a narrow, symmetric emission spectrum, and minimal photo-bleaching. Quantum dot technology, however, is still in its infancy, and problems such as the reproducible manufacture, coating, and derivatization of the nanoparticles continue to hinder development. In addition, although the quantum yield of an individual fluorescent nanoparticle is high, the absolute fluorescence intensity of each particle is low. Recent attempts have been made to increase the fluorescence intensity of the particles by grouping multiple particles into a larger single particle; however, such technology is just emerging (Bruchez et al. (1998) Science 281:2013-2016; Chan et al. (1998) Science 281:2016-2018).
Some compounds containing rare-earth elements such as europium (Eu) are known for their unique optical (fluorescent) properties. Fluorescent nanoparticles comprised of europium compounds are disclosed in U.S. Pat. Nos. 6,010,644 and 6,284,156 to Fu et al. Use of these particles to label biological molecules requires expensive and complex chelation chemistry, and therefore, application of such europium chelates has been limited. Also, the particles disclosed by Fu require the presence of boron during manufacture, which may not be desirable in all instances. For example, although the presence of boron allows the reaction temperature to be lowered one or two hundreds degree (xc2x0 C.), it also results in an increase in the size of the resulting crystals. As fine granularity and small particle size are required in biological applications, particles produced with boron may be unsuitable for biological applications.
Therefore, there remains a need in the art for improved and simplified fluorescent labeling techniques that will provide particles that are suitable for use as labeling agents, and possess high fluorescence intensity. The present invention addresses this need.
In one embodiment of the invention, a boron-free, europium-containing fluorescent nanoparticle is provided. The nanoparticle is comprised of an aluminum oxide based crystal framework that contains a europium activator; at least one energy reservoir selected from the group consisting of Mg, Ca, Sr, and Ba; and at least one co-activator selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Bi.
The particle may be surrounded by a coating that contains functional groups and allows for the attachment of the coated particles to chemical and biological molecules. Accordingly, in another embodiment of the invention a boron-free, europium-containing fluorescent nanoparticle is provided that comprises a core comprised of an aluminum oxide base crystal framework; Eu as an activator; at least one energy reservoir selected from the group consisting of Mg, Ca, Sr, and Ba; at least one co-activator selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Bi; and a coating having functional groups therein.
In another embodiment of the invention, a method for producing such coated, boron-free, europium-containing fluorescent nanoparticles is provided. The method begins by combining aluminum oxide, a europium oxide or salt; at least one salt or oxide of a material selected from the group consisting of Sr, Ca, Mg, and Ba; and at least one co-activator selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Bi, to form a particulate mixture. The combined materials are then heated under a vacuum to provide the europium-containing fluorescent nanoparticle.
If desired, the method may additionally comprise coating the europium-containing fluorescent nanoparticles with a coating composition having functional groups thereon. The coating is generally a silane and the functional groups are selected from the group consisting of primary amino groups, sulfhydryl groups, aldehyde groups, carboxylate groups, alcohol groups, phosphate groups, ester groups, and ether groups, and combinations thereof. The coating step may be carried out by exposing the europium-containing fluorescent nanoparticle and a coating composition to microwave radiation.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.